In the field of microfabrication represented by fabrication of semiconductor devices, a microfabrication technology enabling fabrication with a line width of less than 0.20 μm has been demanded to cope with the recent trend of high integration.
Use of light sources emitting a shorter wavelength light can be mentioned as a method for attaining such miniaturization of patterns. In these days, a KrF excimer laser (wavelength: 248 nm), an ArF excimer laser (wavelength: 193 nm), an F2 excimer laser (wavelength: 157 nm), and EUV (wavelength: 13 nm, etc.) are more abundantly used instead of conventional light sources such as a g-line and i-line.
In regard to the photoresist material used with such light sources emitting a shorter wavelength light, it is difficult for a photoresist containing a novolac resin and naphthoquinone diazido conventionally used with a g-line or i-line to form fine patterns because of formation of tapered patterns due to strong absorption in the deep ultraviolet ray region. Moreover, since this type of photoresist exhibits only a low sensitivity of no more than 1, in terms of a quantum yield, the life of the excimer laser oscillation gas is short in the photo reaction during exposure to light. This causes a problem in the life of a lens, when the lens is used with a photoresist by which the lens is easily damaged by the excimer laser.
As a photoresist suitable for an excimer laser solving these problems, many chemically-amplified photoresists comprising a resin which causes a chemical reaction accompanied by change in solubility in a developer in the presence of an acid catalyst and a photoacid generator which generates an acid upon exposure with light have been proposed.
A widely known typical example of such a chemically-amplified photoresist for a KrF excimer laser comprises a resin containing polyhydroxystyrene in which the phenolic hydroxyl groups are protected by acid-dissociable groups such as an acetal group and t-butoxy carbonyl group and a photoacid generator such as a triaryl sulfonium salt represented by a triphenylsulfonium salt (Japanese Patent Application Laid-open No. 59-45439, for example).
The resin having the polyhydroxystyrene as a base skeleton used for a KrF excimer laser is not suitable as a chemically-amplified photoresist for an ArF excimer laser because of the strong absorption of light at a wavelength of 193 nm. Resin components such as a (meth)acrylate resin having an alicyclic skeleton, a polymer of norbornene derivatives having an alicyclic skeleton in the main chain, and a copolymer of a norbornene derivative and a maleic anhydride have been proposed as a chemically-amplified photoresist for an ArF excimer laser. On the other hand, a triaryl sulfonium salt, when used as a photoacid generator even in a comparatively small amount, reduces radiation transmittance of a resist due to the strong absorption by aromatic rings, even if the above resins are used as a resist. For this reason, the resist has problems in its performance such as difficulty in obtaining high resolution and formation of tapered pattern shape, unless the amount is unduly limited. The triaryl sulfonium salt thus is not always a suitable photoacid generator when an ArF excimer laser is used as radiation.
To overcome the problem of the triaryl sulfonium salt due to its low radiation transmittance, Japanese Patent Application Laid-open No. 2001-354669, for example, proposed other sulfonium salts such as a sulfonium salt containing a 2-oxoalkyl group. This type of sulfonium salt, however, significantly decreases sensitivity although transparency to radiation is remarkably improved. Therefore, practical sensitivity cannot always be obtained. Moreover, when the amount of the additive to improve the anti-basic property of the photoresist or the selection of the additive is not appropriate, the storage stability of the photoresist is impaired.
Japanese Patent Application Laid-open No. 10-232490 discloses a photoacid generator of a cyclic sulfonium salt containing a substituted or unsubstituted naphthyl group and a radiation-sensitive resin composition containing the photoacid generator. The transmissivity of the sulfonium salt having this-type of structure at a wavelength of 220 nm is disclosed. The resist was confirmed to exhibit excellent sensitivity and pattern shape when an ArF excimer laser was used, and to have good storage stability. However, since many of these compounds have an absorption peak near the wavelength of 193 nm, the effect of the absorption on radiation transmittance cannot be neglected when used in a chemically-amplified photoresist to be exposed to an ArF excimer laser. The amount of the compound that can be added is limited. Moreover, cyclic sulfonium salts, particularly those having a sulfur atom directly bonded to the α-position of a naphthyl group, tend to be decomposed by a dark reaction accompanied by a ring-opening reaction due to the steric hindrance of the naphthyl group, thus causing a problem in respect to storage stability of the resist solutions.